1. Technical Field
The present invention is directed to lithographic processes for device and mask fabrication and, in particular, to resolution enhancement techniques for such processes.
2. Art Background
In lithographic processes for device fabrication, radiation is typically projected onto a patterned mask (also referred to as a reticle) and the radiation transmitted through the mask is further transmitted onto an energy sensitive material formed on a substrate. Transmitting the radiation through a patterned mask patterns the radiation itself and an image of the pattern is introduced into the energy sensitive material when the energy sensitive resist material is exposed to the patterned radiation. The image is then developed in the energy sensitive resist material and transferred into the underlying substrate. An integrated circuit device is fabricated using a series of such exposures to pattern different layers of material formed on a semiconductor substrate.
An integrated circuit device consists of a very large number of individual devices and interconnections therefore. Configuration and dimensions vary among the individual devices. The pattern density, (i.e. the number of pattern features per unit area of the pattern) also varies. The patterns that define integrated circuit devices are therefore extremely complex and non-uniform.
As the complexity and density of the patterns increase, so does the need to increase the accuracy of the lithographic tools that are used to create the patterns. The accuracy of lithographic tools is described in terms of pattern resolution. The better the resolution, the closer the correspondence between the mask patterns and the pattern that is created by the tool. A number of techniques have been used to enhance the pattern resolution provided by lithographic tools. The most prevalent technique is the use of shorter wavelength radiation. However, this technique is no longer viable when exposure wavelengths are in the deep ultraviolet (e.g., 248 nm, 193 nm and 157 nm) range. Using wavelengths below 193 nm to improve resolution is presently not economically and technologically feasible because the materials used for lenses in optical lithography cameras absorb this shorter wavelength radiation.
Resolution enhancement techniques (RET) other than simply using shorter wavelength radiation have been proposed. These techniques use exotic illumination from the condenser (e.g. quadrupole illumination), pupil filters, phase masks, optical proximity correction, and combinations of these techniques to obtain greater resolution from an existing camera. However, such techniques typically improve resolution only for some of the individual features of a pattern. The features for which resolution is improved are identified as the critical features. The resolution of many other features is either not improved or actually degraded by such resolution enhancement techniques. Thus, current RETs require a compromise between resolution enhancement for the critical features and resolution degradation for the non-critical features. Such compromises usually require sub-optimal illumination of the critical features in order to avoid significant degradation in the illumination of the non-critical features.
Resolution enhancement techniques have been proposed to customize mask feature illumination in projection lithography for the various different features in the mask. One such technique is described in Matsumoto, K., et al., "Innovative Image Formation: Coherency Controlled Imaging," SPIE, Vol. 2197, p. 844 (1994). That technique employs an additional mask and additional lens to customize the radiation incident on each feature of the mask. A similar system is described in Kamon, K., "Proposal of a Next-Generation Super Resolution Technique," Jpn. J. Appl. Phys., Vol. 33, Part 1, No. 12B, p. 6848 (1994).
In the resolution enhancement techniques described in Matsumoto et al. and Kamon et al., the first mask has features that are identical to the features on the second mask. The features on the first mask diffract the radiation incident on the mask, and the diffracted radiation illuminates the identical feature on a second mask. For example, radiation transmitted through a grating pattern on the first mask is projected onto an identical grating pattern on the second mask. Similarly, radiation transmitted through an isolated line on the first mask is projected onto an identical isolated line on the second mask. When the diffracted energy from the first mask illuminates the identical feature on the second mask, the resulting image is often superior to an image obtained from quadrupole illumination of the pattern. Therefore, this resolution enhancement technique provides an improvement in aerial image contrast (i.e. the image in the focus plane of the projection lens) over conventional off-axis illumination using the quadrupole system.
The above-described resolution enhancement technique provides customized illumination for more features than quadrupole illumination. However, the above-described technique does not improve the resolution of all features in the pattern. Furthermore, the two-mask system is costly and complex. Specifically, the system requires two precisely patterned masks instead of one. The corresponding features on the first and second masks must match precisely. The alignment of the first and second masks is also critical. Furthermore, the technique is limited because the features on the first mask are illuminated uniformly. Thus, the problems associated with non-customized illumination of a patterned mask are not eliminated by this system, but simply stepped further back into the optics of the system. Therefore, resolution enhancement techniques that improve the resolution of all features and are cheaper and easier to implement are sought.